(CBS/AP) WASHINGTON - The FBI released surveillance video of Navy Yard shooter Aaron Alexis and reiterated that he didn't appear to have targeted any specific individuals when he opened fire inside a building, killing 12 people.

Valerie Parlave, head of the FBI's field office in Washington, said Wednesday during a news conference that investigators are continuing to explore the background and motivations of 34-year-old Aaron Alexis. However, she says Alexis had a well-documented history of mental health issues.

Parlave says Alexis held a delusional belief that he was being controlled by extremely low frequency waves, or ELF waves. Etched into the left side of the receiver of the shotgun he used in the attack were the phrases "Better off this way!" and "My ELF weapon!" Etched into the barrel of the shotgun were the words "End to the torment!" Etched into the right side of the shotgun receiver were the words "Not what yall say!"

Alexis, a former Navy reservist and IT contractor, was killed in a shootout with police.

The FBI released the following timeline, detailing Alexis' activity during the morning of September 16, 2013:

7:53 a.m. - Alexis' rental car, a blue Toyota Prius with New York plates, entered Parking Garage #28 at the Washington Navy Yard, located directly across from Building #197.

8:09 a.m. - Alexis exited the elevator on the fourth floor and entered the men's bathroom carrying a backpack and a clipboard.

8:15 a.m. - Alexis crossed the hallway into the 4 West area of Building #197 with shotgun, but without the backpack or a clipboard.

8:16 a.m. - Alexis shot the first victim in the 4 West area of Building #197.

8:17 a.m. - First 911 call is received from the fourth floor of Building #197.

8:20 a.m. - Alexis left the fourth floor using the stairs and entered the third floor.

8:28 a.m. - Alexis appeared on the first floor.

8:57 a.m. - Alexis returned to the third floor.

9:25 a.m. - Law enforcement shoot and kill Alexis on the third floor.

The weapons recovered at the scene are being examined still, but the FBI did say the shotgun used at the scene had been modified with a sawed-off barrel, and had purple duct tape covering the end.

Alexis backpack was found to have included a roll of purple duct tape and "electronic media." The FBI said the media "revealed several relevant communications," although they did not elaborate, except to say the communications had indicated "that Alexis was prepared to die during the attack and that he accepted death as the inevitable consequence of his actions."

The FBI also said a document retrieved from his electronic media stated: "Ultra low frequency attack is what I've been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this."